Calculate the flux of the current model between certain limits.
XSPEC> flux [<lowEnergy> [<hiEnergy>]]
where <low Energy> and <hiEnergy> are the values over which the flux is
calculated. Initial default values are 2 to 10 keV. The flux is given in
units of photons/cm^2/s and ergs/cm^2/s. The energy range must be contained
by the range covered by the current data sets (which determine the range over
which the model is evaluated). Values outside this range will be
reset automatically to the extremes. Note that the energy values are two
separate arguments, and are NOT connected by a dash (see parameter ranges in
the freeze command).